Composition and method for coating metal surfaces



United States Patent C 3,272,664 CUMPOSlTiON AND METHOD FOR COATINGMETAL SURFACES lLouis Schlossberg, Oak Park, Earl Eugene Logue,Birmingham, and Paul J. Carrier, Detroit, Mich., assignors to DetrexChemical Industries, Inc., Detroit, Mich, a corporation of Michigan NoDrawing. Filed July 25, 1963, Ser. No. 297,694

6 Claims. (Cl. 148-617) This invention relates generally to compositionsand methods for producing conversion coatings on ferrous metal surfaces.More particularly this invention relates to compositions and methods forproviding what are known as amorphous or micro-crystalline coatings ofexceptional quality and weight composed predominantly of ironphosphate-iron oxide mixed crystals.

Conversion phosphate coatings applied from aqueous phosphate solutionshave previously been known and used in the metal treating art. Numerousprocesses have been proposed, yielding coatings of widely varyingproperties. While the compositions and processes of the prior art haveprovided coatings which are satisfactory in some cases, it has beenfound that there are disadvantages inherent in such compositions andprocesses, particularly under conditions of actual commercial use whererela tively heavy coatings are desired. Included among suchdisadvantages are the formation of appreciable amounts of sludge, thenecessity of utilizing a run-in period or initial aging, the difiicultyof consistently forming a heavy coating, and the formation of excessiveamounts of undesirable loose deposit on the surface of the coating,which in the industry is referred to as dust.

Therefore, it is a primary object of this invention to produce a premiumquality conversion coating of the type which is known in the art as aniron phosphate coating and which possesses the superior attributes nowexpected of such premium coatings for paint adhesion, salt-sprayresistance and humidity resistance, while at the same time consistentlyproviding heavy coatings.

It is a further object of this invention to provide a process ofproducing such coatings without the formation of appreciable sludge.

It is a further object to provide an improved process wherein the run-inperiod or initial aging of a fresh bath can be eliminated or greatlyabbreviate-d and still achieve results comparable to those from awell-aged, smoothly operating bath.

It is a still further object to provide an improved composition andprocess which produces consistently heavier coatings than do presentlyavailable processes, under normal operating parameters, thus assuringthe user of a Wider safety factor for meeting the coating weightrequirements of his customers.

A further object is to provide an improved liquid concentrate whichfacilitates handling and additionally offers less hazard because of agreatly reduced oxidizer content, while improving the coating activityof the solution.

A still further object is to provide a process which precludes thenecessity for seeding the process solution with almost immeasurable butnecessary amounts of ferrous ion in order to achieve heavy coatingweights on steel in the order of 45 milligrams per square foot andhigher.

A further object is to provide an improved composition and process whichproduces substantially less dust on coated parts than is now possible.This has the decided advantage of reducing handling problems andcontamination in subsequent operations.

Other objects will become apparent from the description of thecomposition and process set forth below.

3,272,664 Patented Sept. 13, 1966 ICC? It has been known in the priorart that dihydrogen phosphates of the alkali metals, including ammonium,may be caused to produce phosphate coatings on metals, under properoperating conditions, by the addition of an oxidizing radical, such asthe C10 ion, provided at least 0.5% of oxidizing radical up to about 4%oxidizing radical is included in the bath. It has also been known andestablished that a reduction in total chlorate ion by as little as 0.1%below the minimum of 0.5% results in a working bath which does notperform effectively, either the desired quality and degree of coating orthe general operability of the coating bath. Other agents have beenproposed in place of chlorate, such as nitrite, bromate, and sulfiteions. However, effective results in the use of such compounds have beenlimited to amounts at least equivalent in activity to 0.5 by weight ofchlorate ion.

In the development of the present invention, we have discovered ahitherto unsuspected synergistic relationship between chlorate anddinitrobenzene sulfonic acid (specifically 2,4 dinitrobenzene sulfonicacid or its sodium salt) when used in combination in dihydrogenphosphate coating baths, Which makes possible a highly functionalcombined total additive content substantially below 0.5%. Thissynergistic relationship is evidenced by the fact that the addition ofeither compound separately to dihydrogen phosphate coating baths in anamount corresponding to 0.4% and substantially less gives unsatisfactoryresults, whereas the addition of a combination of the two compounds inan amount totaling 0.4%, or even less, provides phosphate coatingresults which are measurably superior to phosphate coatings producedthrough the use of 0.5 or more of chlorate ion both as to quality andcoating weight. As a result of this synergistic action, very superiorand unexpected coatings are achieved at greatly reduced over-allconcentrations, so that it is possible, in the practice of the presentinvention, not only to obtain greater coating weights than werepreviously possible but also :to avoid some of the disadvantages whichapparently stem from the greater total amounts of oxidizing agentspreviously found to be necessary, particularly where heavy coatings aredesired.

The following examples are set forth as illustrative of the presentinvention, but it should be noted that the specific formulations ofconcentrates and working solutions hereinafter set forth are intendedmerely as examples of the present invention. The relative amounts of thevarious ingredients may be varied over a wide range, within which thebenefits of the synergistic relationship of the additive compounds areobtained. However, for the formation of heavy coatings mgs. per sq. ft.or higher) essentially free of dust, the preferred range of chlorate ionis from 0.3% to 0.4% and the range of dinitrobenzene sulfonate is from0.002% to 0.4%. In the examples the sodium salts of the phosphate,chlorate and nitrobenzene sulfonate ingredients have been employed. Foreach ingredient any of the other alkali metal salts may be used, but thesodium salts are preferred as a matter of economy. The pH range ispreferably from about 4.2 to about 6.

Water 56.50

To prepare a working solution containing 0.38% of C10 ion, 5.25% of theabove concentrate was added to 94.75% of water. The pH was adjusted withsoda ash to approximately 5.0. The solution was then aged by sprayprocessing steel to the extent of 20 square feet of steel surface areaper gallon of solution. The chlorate ion level was maintained at 0.38%at all times.

Standard mild steel panels were then spray coated with this solution atnormal operating conditions of one minute spray exposure, at 160 F. and10 psi. After spray coating, several panels were checked for coatingweight using a standard chromic acid stripping technique. The coatingweight was 43 mgs. per square foot.

Several of the phosphate coated panels were also painted by immersion ina standard epon base appliance paint, the average paint thickness afterbaking being about 0.4 mils. The painted panels were subsequentlyscribed and then submitted to salt spray test exposure. Failure, whichwas recorded when one-quarter inch peel was evident from the scribe lineand/or when heavy blisters occurred, took place in six days. This saltspray performance Was rated as poor.

Example 2 A working solution was prepared essentially identical to thatof Example 1 except that the sodium chlorate was omitted and in itsplace 0.4% of 2,4 dinitrobenzene sodium sulfonate was added. Thesolution was pH adjusted and aged in the same manner, as in Example 1.Then standard steel panels were spray coated, painted, scribed andtested as in Example 1. The coating weight was 78 mgs. per sq. ft. Thetime to failure in the salt spray cabinet was 13 days. This salt sprayperformance, though superior to that of Example 1, was not suflicientlygood. Moreover, panels phosphated with this working solution showedexcessive amounts of undesirable dust residues.

Example 3 A concentrate was made up as follows:

Percent by weight Monosodium phosphate 25.00 Phosphor acid, 75% 9.25Sodium chlorate 9.25 2,4-dinitrobenzene sodium sulfonate 0.50

Water 5600 To prepare a working solution containing 0.37% of C10 ion and0.025% of 2,4 dinitrobenzene sodium sulfonate, 5.1% of the aboveconcentrate was added to 94.75% of water. The solution was then pHadjusted, aged and panels treated as in Example 1. It was found that thecoatings produced were of a very high degree of quality, fine grained,and essentially free of dust. The coating weight was 72 mgs. per sq. ft.Salt spray performance was excellent, no failure taking place even after17 days of exposure.

Example 4 A concentrate was prepared to conform to the compositiontaught in US. Patent No. 2,609,308, as follows:

Percent by weight Monosodium phosphate 25.00 Phosphoric acid, 75% 9.25Sodium chlorate 12.30 Water 53.45

To prepare a working solution containing 0.5% C10 ion, 5.25% of theabove concentrate was added to 94.75% of water. The solution was pHadjusted and aged in the same manner as Example 1. Standard panels werespray phosphated, painted, scribed and tested as in Example 1. Thecoating weight obtained was 50 mgs. per sq. ft. Salt spray resultsindicated only 12 days to failure.

Example A working solution was prepared essentially identical to that ofExample 3 except that the solution contained 0.38% C ion and 0.05% 2,4dinitrobenzene sodium sulfonate rather than 0.38% C10 ion and 0.025% 2,4dinitrobenzene sodium sulfonate. The solution was then pH adjusted andaged and steel panels were spray coated, painted and scribed as inExample 1. The coating weight was 70 mgs. per sq. ft. The time tofailure in the salt spray cabinet was over 17 days. Coatings were ofexcellent quality and free of dust.

In the practice of the present invention, the working solution isapplied to the metal surfaces by either spray or immersion. Theinvention is particularly useful in what are known as power spraywashers through which the metal work is conducted on a conveyor system.Time cycles of approximately 30 seconds to seconds duration aresatisfactory for spray processes, while immersion applications requirefrom about 3 to 5 minutes of processing time. The preferred operatingtemperature is approximately -165 F., although somewhat lower or highertemperatures may be used for specific purposes.

While greatly improved results are achieved in practicing the presentinvention through a broad range of concentrations of synerigsticadditives, we have found that the particular value of the presentinvention resides in the fact that heavy coatings in the order of 70milligrams per square foot can be achieved having enhanced paintadhesion characteristics and at the same time the total concentration ofsynergistic additives may be held to levels substantially lower thanheretofore employed.

It will be understood that substitution of equivalent materials andvariations in the amounts of the materials employed are included withinthe present invention, as defined in the claims set forth below.

Having described our invention, we claim:

1. A solution for producing phosphate coatings on ferrous metal surfacesconsisting essentially of water, an alkali metal acid phosphate, analkali metal chlorate and an alkali metal dinitrobenzene sulfonatesynergistically cooperative with said chlorate.

2. A solution for producing phosphate coatings on ferrous metal surfacesconsisting essentially of water, an alkali metal acid phosphate, analkali metal chlorate and an alkali metal dinitrobenzene sulfonate in anamount synergistically cooperative with said chlorate to cause saidsolution to produce heavy, substantially dust free coatmgs.

3. The solution of claim 2 in which the alkali metal chlorate is presentin the range from 0.30% to 0.4% and the alkali metal dinitrobenzenesulfonate is present in the range from 0.002% to 0.4%.

4. A solution for producing phosphate coatings on ferrous metal surfacesconsisting essentially of water, sodium acid phosphate, sodium chlorateand 2,4 dinitrobenzene sodium sulfonate synergistically cooperative withsaid chlorate.

5. The process of producing a phosphate coating on a ferrous surfacecomprising applying to the ferrous surface a solution consistingessentially of water, alkali metal acid phosphate, alkali metal chlorateand an alkali metal dinitrobenzene sulfonate synergistically cooperativewith said chlorate.

6. The process of producing a phosphate coating on a ferrous surfacecomprising applying to the ferrous surface a solution consistingessentially of water, alkali metal acid phosphate, alkali metal chlorateand 2,4 dinitrobenzene sodium sulfonate in an amount synergisticallycooperative with said chlorate to cause the formation of a heavyphosphate coating.

References Cited by the Examiner UNITED STATES PATENTS 2,375,468 5/1945Clifford et al. 1486.15 2,609,308 9/1952 Gibson et al. 1486.15 X2,657,156 10/1953 Hyams et al. 1486.17 X

ALFRED L. LEAVITT, Primary Examiner.

R. S. KENDALL, Assistant Examiner.

1. A SOLUTION FOR PRODUCING PHOSPHATE COATING ON FERROUS METAL SURFACESCONSISTING ESSENTIALLY OF WATER, AN ALKALI METAL ACID PHOSPHATE, ANALKALI METAL CHLORATE AND AN ALKALI METAL DINITROBENZENE SULFONATESYNERGISTICALLY COOPERATIVE WITH SAID CHLORATE.